This all metal stereoscope is called a Standard II and
was manufactured in Germany by a company called Rada-Metallwarenfabrik,
based in Frankfurt. The viewer dates to around 1955.

It has ground glass lenses and gives a magnificent
bright, clear image. It was designed to take two sizes of slide, 45
x 101 mm and 41 x 101mm (Belplasca format). The Standard II could be
focused by means of a wheel located between the lenses. This particular
model has a focal length of 50mm, which can be seen stamped on the
right-hand side.